Voc vs Vmpp Panel Voltage

ligwyd

Voc (Open ciruit voltage)

Vmpp (Voltage at max power point)

When adding voltages to calculate max string voltage going into a charge controller, can you use Vmpp or must you use Voc at all times?

Is it safe to just add up Vmpp and X by 1.25 to account for cold and sunny days? Whats the rule of thumb?

mike95490

You always use Voc and adjust it upwards for the historic cold for the site. 

Generally, the MPPT mfg has a "String Sizer Tool" that your put your panel info into and the min temperature
and it will report if the proposed configuration is acceptable or if it will violate the warranty.

In a pinch, you could use this, and massage the results to your device:
http://www.midnitesolar.com/sizingTool/index.php
  it will at least give you the range of voltages to expect under worst case conditions.